The natural flame instability in any flare or furnace produces noise. In rare cases, the noise can be loud enough to damage equipment, cause nuisance shutdowns or bother the neighbors. Simple hardware changes can eliminate the problem, but sometimes finding the right fix takes time.

Figure 1. A gas flame can be distorted by resonant combustion noise inside a furnace. The frames show one complete cycle of the sound wave.
Courtesy of Cambridge University Engineering Department

All flames make noise -- and larger ones make more. Most people can’t hear the noise from a pocket lighter, but they know when their gas water heater or gas-fired central heat burner starts. And any industrial-size furnace makes enough noise that you can tell if it is on without looking. Usually, combustion noise can be ignored -- it seldom causes problems. In rare cases, though, it is loud enough to draw complaints from the neighbors and even vibrate equipment to destruction. This article will consider five cases like that.

First, A Little Theory

Any noise has a frequency and an amplitude.

You can hear noise over a frequency range of about 20 to 20,000 Hz, or cycles per second. Noise at a frequency less than 20 Hz might be felt but not heard: You might notice the floor or windows rattling even though you cannot hear the sound. If you have good hearing, the quietest sound you can hear is slightly louder than 0 decibels (db). The loudest (I’m told) is 120 db, at which point your ears are quickly damaged.

In the combustion world, there are two categories of noise:
  • Noise formed by the resonant characteristics of ducting, stacks, ovens, furnaces, etc. The trigger for this noise might be a flame or some other source, but the vessel happens to be just the right size and shape to reinforce and amplify. The burner flame then might add energy to the sound, and the result can be startling.
  • Noise formed by a flame only (due to burner instability). This type of noise usually is not very loud, but it increases as the pressure and flow of fuel increase.
Figure 1 shows how a gas flame can be distorted by resonant combustion noise inside a furnace.

Figure 2. Installed at a refinery in Houston, the Claus tail gas incinerator included a relatively long tail gas duct running from the last sulfur condenser down to the natural gas-fired incinerator burner. A centrifugal air blower was installed to overcome the pressure drop across a watertube waste heat boiler positioned at the furnace outlet.
Courtesy of Callidus Technologies

Case History 1: Claus Tail Gas Incinerator

A Claus tail gas incinerator was built at a refinery in the Houston area. It included a relatively long tail gas duct running from the last sulfur condenser down to the incinerator burner. The burner was natural gas fired and included a centrifugal air blower to overcome the pressure drop across a watertube waste-heat boiler positioned at the incinerator furnace outlet and exhausting to a carbon steel stack. This system was designed for future plant expansion, so throughput was only a fraction of the design (figure 2).

Immediately upon startup, the operators heard a very loud, low frequency noise or pulsation. The noise disappeared when the burner was fired at its design rate, and, of course, it went away when the burner was off.

Attempts to modify the burner fuel gas injection geometry had little effect on the noise, even though this step often eliminates or at least reduces similar problems. Among the changes tried were shifting the fuel gas gun tip position in the burner tile and changing the fuel gas tip drill pattern.

An acoustics expert was called in. Microphones were installed at several locations throughout the system, but the problem source remained elusive until, with the burner turned off, low level noise at the problem frequency was detected. This meant that the noise was being triggered somewhere in the system, and the burner was simply amplifying the signal into a vessel accidentally sized for resonance.

Ideas to alter the resonant frequency of the equipment were considered. For instance, shortening or lengthening the stack would have changed the resonant point and cut the noise amplitude. The fix selected consisted of installing a perforated stainless steel plate across the boiler outlet flange. The perforations were sized to create as much flue gas pressure drop as the blower and Claus plant could handle. That fixed the noise problem by interrupting the resonance.

Figure 3. The thermal oxidizer system was designed to be used with a waste heat boiler, so the horizontal furnace was connected via a long duct to the exhaust stack. A forced-draft natural gas burner was used to bring the furnace up to temperature.

Case History 2: Chemical Plant Thermal Oxidizer

At a chemical plant also near Houston, a thermal oxidizer system was installed to dispose of offgas from an acrylonitrile plant. The system was to have a waste-heat boiler installed later, so the horizontal furnace was connected via a long duct to the exhaust stack. A forced-draft natural gas burner was used to bring the furnace up to temperature (figure 3).

Once the refractory was cured out, the operator switched the vent gas stream away from the atmospheric vent and into the burner. Immediately a very loud, high pitched noise was heard. Apparently, the furnace system was sized just right for resonance, and the burner provided the amplification, while the noise was started at the inlet valve.

Fortunately, a simple change to the inlet valve position changed the frequency enough to reduce the noise to acceptable levels. In this case, the change was easy to accomplish and worked “like magic.”

Case History 3: Elevated Flare

A refinery in California installed a large, steam-assist, elevated flare. Steam injection was necessary for smokeless burning of the waste gases (figure 4). Upon startup, a loud, low frequency (2 Hz) noise was produced. It resulted in complaints of vibrating walls and garage doors from neighbors well away from the flare site, and steps were immediately taken to find the cause and eliminate it.

With trial and error and a few lucky guesses, the cause was determined to be the steam jet position in relation to the waste gas injection passage. By raising the steam jet elevation about 2", the problem noise was eliminated and smokeless operation maintained.

Figure 4. The position of the steam jet on the steam-assisted, elevated flare caused low resonance noise. Moving the steam-jet tip up 2" higher eliminated the problem noise.

Case History 4: North Dakota Hydrogen Vent Incinerator

A petrochemical plant in North Dakota produced a waste gas rich in hydrogen. It was to be burned in a forced-draft incinerator. The incinerator floor was equipped with a number of floor-mounted burners, each with a central fuel gas gun mounted inside a refractory throat. Upon startup, a very loud “hum” was produced. In this case, there were operator complaints, and the amplitude was so great that it was loosening the nuts holding the furnace platforms and other hardware in place. With nuts falling, the furnace could not be operated while personnel were below.

The problem was solved by adding a central gas passage through the center of each burner gas gun. This reduced the waste gas pressure drop at the gun tip and changed the flame geometry enough to “detune” the burners and reduce the noise to acceptable levels. The changes delayed mixing of the waste gas with the combustion air, but the increased flame length was not a problem.

Figure 5. The incinerator floor was equipped with a number of floor-mounted burners, each with a central fuel gas gun mounted inside a refractory throat.
Courtesy of Zeeco

Case History 5: Chemical Plant Waste Heat Recovery

A petrochemical plant near Houston operated a large, horizontally fired incinerator mated to a watertube heat recovery boiler and economizer. The flue gas was ducted to atmosphere through a 100' stack close-coupled to the economizer. The waste gas had very low heating value and was injected, along with the combustion air, through a series of stainless pipes installed through the furnace refractory lining. A natural gas burner started the combustion and two waste liquids were sprayed in between the burner and the waste gas injectors.

With increased plant production, a noise began to appear. In this case, the frequency was only one or two cycles per second, and was detected by rhythmic swelling of the fabric expansion joints connecting the economizer to the stack. Expansion joint life was reduced to about six months, and the system had to be taken down for fabric replacement (total shutdown lasting approximately three days each time). Previously, the plant could be run nearly two years between shutdowns.

In this case, modifying the waste distribution across the pipe injectors resulted in enough amplitude reduction to solve the problem. The low frequency noise was still present, but flexing of the expansion joint fabric was eliminated.

More Theory

How can a fuel gas burner act as an amplifier for noise? It appears the pressure changes that cause the noise are created when the fuel and air streams are combined improperly. If the two streams are combined incorrectly, pockets of rich or lean mixtures are created which are not flammable at the operating conditions. As additional fuel or air enters the pocket, the mixture becomes flammable and burning continues, increasing the volume of the pocket, which displaces the air and fuel streams, forming a nonflammable pocket again. As this cycle continues, the on/off nature of the combustion is seen as pressure fluctuations. With many industrial size furnaces, the pulsation frequency is in the range of one or two per second, but with hydrogen-rich fuel gas or fuel gas under high pressure, the frequency can be greater.

All burners are equipped with a special alloy tip or a refractory tile to anchor the ignition point. If the anchor method is insufficient, the flame may initiate first at one point and then jump to another point, giving rise to pressure fluctuations and noise.

Larger burners typically are diffusion-type designs, meaning that the fuel (gas, oil or pulverized coal) is injected adjacent to the combustion air stream. As the fuel diffuses into the air stream (and vice versa), the burner flame develops and grows until all of the fuel is oxidized.

So, how would a fuel burner amplify an existing noise, as apparently happened in the field cases cited? In the first example with the Claus tail gas incinerator, the inert waste gas was injected through an annular gap surrounding the flame zone. Apparently, the pressure fluctuations in the waste gas distorted the fuel gas/air mixing process at the existing frequency, and the energy already available from the flammable mixture pockets was redirected from standard combustion noise to the new frequency, which happened to be the same as the resonant frequency of the furnace system. Bad luck. The other cases involved a similar effect, although the circumstances look quite different.

In conclusion, if your installation has unacceptable noise, remember that the negative effects of combustion pulsation include:
  • Rhythmic flame body displacement. For example, the flame scanner may lose sight of the flame and trigger nuisance shutdowns.

  • Brief loss of combustion airflow as oven or furnace pressure peaks, reducing airflow to the burner. In these cases, the low flow switch may trip, again causing nuisance shutdowns.

  • “Breathing” of the oven or furnace shell and expansion joints, leading to material failure through fatigue.

  • Structural damage to the oven or furnace if the amplitude is high enough.

  • Extremely irritated operators and neighbors.
Eliminating combustion pulsation and excessive noise is possible through changes in burner operation (reduce or increase airflow, change steam flow, etc.), but often, some sort of hardware change is required. There is some hope from acoustic analysis of the resonating chamber, but this is presently very difficult and typically too expensive to use in the equipment design phase.

More practical approaches include changing the location or velocity of fuel injection. Another involves changing the location direction of combustion airflow, steam flow or waste flow. And in some cases, steps must be taken to change the resonant frequency of the furnace/stack assembly. These changes are typically accomplished in an experimental manner. Sometimes, observation of the flame body will give a clue as to the best approach.